An image reflected in a mirror may appears in the visual field in a location opposite to the physical location of the object in the environment, in particular it may appear on the right when the object is on the left and vice versa. Through experience people have knowledge that reflections are not real objects an also that they are informative about object locations. We tested the importance of this knowledge in a simple task in which participants had to respond to a visual stimulus (light from an led) that could appear in the right or left visual field. Participants had to respond with the contralateral hand, i.e. they had to press the right button if the light was on the left and vice versa. An irrelevant sound could also originate from the left or the right side. To avoid different bouncing of the sound the mirror condition was compared to a glass condition so that the solid surfaces were identical in size. When light and sound had the same spatial origin responses were faster, however this was only true as long as the light was seen though a glass and not seen reflected in a mirror. We conclude that participants are influenced by the knowledge about the origin of a light when when this information is irrelevant for their task.